Polyarylene sulfide (hereinafter referred to as PAS for short), typified by polyphenylene sulfide (hereinafter referred to as PPS for short), has properties suitable for engineering plastics, such as excellent heat resistance, barrier properties, chemical resistance, electrical insulating properties, and moist-heat resistance, and has been used mainly in injection molding and extrusion molding applications and in various electrical and electronic parts, machine parts, automotive parts, films, fibers, and the like. Especially in recent years, downsizing and weight saving of molded articles have been accelerated for the purpose of environmental load reduction and cost reduction, whereby the shapes of molded articles have been increasingly complicated. Although a PAS having high melt flowability which can be applied to the complicated-shaped molded articles has been demanded, a PAS having high melt flowability tends to have an increased volatile component content during melting because it generally contains a large amount of low-molecular-weight components. Such a volatile component causes stains of a metal mold and vent clogging of a metal mold to thereby cause a problem of reduced production efficiency, therefore a PAS having both high melt flowability and a low volatile component content during melting has been strongly demanded. For further environmental load reduction and cost reduction, improved productivity by shortening a molding cycle has been demanded, and a PAS having a higher melt crystallization temperature has been desired. The present invention includes the discovery of a PAS combining the elements which have been strongly demanded for injection molding applications, that is, high melt flowability, a low volatile component content, and a high melt crystallization temperature, and a method for producing the same.
As a representative method for producing a PAS, the method in which a sulfidizing agent such as sodium sulfide is reacted with a dihalogenated aromatic compound such as p-dichlorobenzene in an organic polar solvent such as N-methyl-2-pyrrolidone (hereinafter referred to as NMP for short) is known. As a method for reducing the content of volatile components which generates when a PAS is melted, the method in which a PAS slurry resulting from the reaction is washed with an organic solvent such as acetone or NMP is commonly known. However, in this method, not only low-molecular-weight substances, a principal component of the volatile component, but also oligomers are removed by washing, and therefore the flowability when the PAS is melted tends to decrease. Therefore, investigations have been carried out to reduce the volatile component content while containing a certain amount of oligomers and maintaining high melt flowability. For example, Patent Document 1 discloses a method in which a polyhalogenated aromatic compound is reacted with a sulfidizing agent in a polar organic solvent; the reactant is recovered by the flash method; the resulting polymer is acid-treated at pH of 2 to 8 and 80 to 200° C.; and the resultant PPS is heat-treated under an atmosphere having an oxygen concentration of not less than 2% by volume at 160 to 270° C. for 0.2 to 50 hours. Generally, it is known that heat-treatment (cure) of a PPS under an oxygen atmosphere causes an increased melt viscosity due to oxidation crosslinking and decreased melt flowability, but in Patent Document 1, high melt flowability and a low volatile component content were achieved at the same time by controlling the heat treatment conditions at certain conditions. However, the peak temperature of crystallization of a PPS when cooled from a molten state under conditions of 20° C./min using differential scanning calorimetry (hereinafter referred to as DSC for short) was not more than 240° C., and a PPS having a high melt crystallization temperature suitable for injection molding was not obtained.
As a method for obtaining a PAS having a high melt crystallization temperature, acid-treating in washing after the reaction is known. For example, Patent Document 2 discloses a method in which sodium hydrosulfide is reacted with p-dichlorobenzene in the presence of NMP at 220° C. for 1.5 hours and further at 245° C. for 2 hours, and an acid treatment and an alkali treatment are combined under certain conditions in a post-treatment step of the resulting polymer. Although this method indeed provided a PAS having a high melt crystallization temperature, the PAS having a peak temperature of crystallization of not less than 248° C. when cooled from a molten state under conditions of 20° C./min using DSC was not obtained.
As an alternative method of increasing the melt crystallization temperature of PAS, Patent Document 3 discloses a method in which an alkali metal sulfide or an alkali metal sulfide-forming compound is reacted with a dihalogenated aromatic compound in an organic amide solvent at a temperature of 160 to 290° C.; the resulting reaction mixture is subjected to solid-liquid separation; the separated solid content is washed with an organic amide solvent; the separated solid content is further slurried with an organic amide solvent; and the slurry is reacted again at 160 to 290° C. for 10 minutes to 30 hours. Patent Document 4 discloses a method in which an alkali metal sulfide is reacted with a dihalogenated aromatic compound in an organic amide solvent at 260° C. for 2 hours; and 0.1 to 48 mol. % of oxygen per mol of an alkali metal sulfide charged is added to the resulting PAS slurry after completion of the reaction. Although these methods indeed provided a PAS having such a high melt crystallization temperature that a peak temperature of crystallization when cooled from a molten state under conditions of 10° C./min using DSC is not less than 250° C., both had a high melt viscosity and was not suitable for injection molding applications requiring high flowability.
Patent Document
[Patent Document 1] JP 2009-144141 (claims, examples)
[Patent Document 2] JP 8-198965 (claims, examples)
[Patent Document 3] JP 2-102228 (claims, examples)
[Patent Document 4] JP 2001-172387 (claims, examples)